1. Field of the Invention
The present invention relates to dry etching, more particularly to a process for the dry etching of a metal film which comprises aluminum as a main component and is provided on a semiconductor chip.
It is preferable to apply the present invention to formation of a conductor pattern from an aluminum film or an aluminum based film in the production of semiconductor devices.
2. Description of the Prior Art
Materials of a conductor pattern of a semiconductor device are generally aluminum or aluminum based metal, such as aluminum-copper, aluminum-silicon or aluminum-copper-silicon. The addition of copper to aluminum increases its electromigration resistance, and the addition of silicon to aluminum prevents silicon depletion of shallow junctions.
In a case where a conductor pattern of a semiconductor device is formed by etching an aluminum film or an aluminum based film, dry etching, i.e. plasma etching, reactive ion etching, or reactive sputter etching, is preferred to wet chemical etching for several reasons. For example, dry etching is cleaner, results in a smaller linewidth of the pattern (i.e. film pattern), there are no noxious and polluting acids and solvents involved, and it is more economical than wet etching. Several etchant gases, for example, the following three kinds of gases, have been proposed for the dry etching of an aluminum film and an aluminum based film.
(1) Mixed gas of carbon tetrachloride (CCl.sub.4) and inert gas (He, Ar) PA1 (2) Mixed gas of phosphorus trichloride (PCl.sub.3) and boron trichloride (BCl.sub.3) PA1 (3 ) Boron trichloride (BCl.sub.3) with or without oxygen (O.sub.2)
When dry etching of an aluminum film using this mixed gas is finished, a so-called "aluminum residue", which is aluminum oxide, tends to remain.
When an aluminum-copper film is dry etched with this mixed gas, the aluminum of the film is converted into aluminum trichloride (AlCl.sub.3), which is volatile at approximately 130.degree. C., and therefore, aluminum removal occurs. However, the copper compound of the film is volatile only at temperatures above 1000.degree. C. and, since the temperature does not exceed 150.degree. C. during the etching period, volatilizing of the copper compound does not occur. As a result, since copper residues accumulate during the dry etching and prevent further etching, it is difficult to etch an aluminum-copper film.
This mixed gas (PCl.sub.3 +BCl.sub.3) has the ability to cause chemical sputtering of a metal film to be etched and, therefore, it is possible to etch an aluminum film and an aluminum-copper film, and simultaneously, to remove aluminum residues and copper residues. However, phosphorus trichloride has strong corrosive tendencies, easily reacts with moisture in the air to produce hydrochloric acid (HCl), and is labile. Furthermore, the permissible concentration range of the phosphorus trichloride is very narrow and depends on the dry etching conditions, such as the pressure inside the vaccum system, the power of the etching apparatus and the total pressure of the mixed gas. If the concentration of the phosphorus trichloride is outside the permissible range, satisfactory etching can not be performed. In addition, when an aluminum film or an aluminum based film is dry etched using this mixed gas, the mixed gas tends to undercut a masking film (i.e. a patterned photoresist film), namely, the etchant gas horizontally etches a portion of the aluminum or aluminum based film which is covered with the masking film.
When an aluminum film is etched using boron trichloride only, the aluminum etchrate (i.e. etching rate) is small, since the number of generated chloric radicals is small. The addition of oxygen to the boron trichloride increases the number of the generated chloric radicals, so that the aluminum etchrate increases, but undesirable undercutting occurs.
As mentioned above, each of these etchant gases has demerits.
Furthermore, a person having an ordinary skill in the art might consider using fluorine gas, being one of halogen gases, as the etchant gas. However, on an aluminum film to be etched, an aluminum fluoride film is gradually formed since the aluminum fluoride is not volatile at temperatures under 1260.degree. C. The aluminum fluoride film serves as an inert film, so that further etching progress is not made.